I do delts 90% of the time, and I aim for the rear head (least pain). My secret is not to do it after a workout, as I want to be as limber as possible so as to reach. I hold the syringe like a pen, and stab, then aspirate with my thumb and inject with my index finger. The other secret is to use a BIG needle (22g or bigger). Unless you're into finger-powerlifting, you're going to have a tough time pushing the plunger with a small ga needle. I use a 21 x 1 1/4 or even 1 1/2 inch, and honestly it feels the same as a 23 or even 25. I have big delts, so use a shorter needle if you're less than big in the delts, or you'll risk injecting the rotator cuff, and that would be a BAD thing.
Charles